This invention concerns improvements in or relating to optical filters and more particularly to wavelength selective reflective filters which can be used, in particular, in multiplexing or demultiplexing systems. The invention relates to apparatus, and more particularly demultiplexing and multiplexing apparatus, employing such filters.
It has previously been proposed to provide in a multiplexer/demultiplexer a reflective grating arranged so that, in demultiplexing mode, a beam of incident light having a plurality of wavelengths impinges on the grating to be reflected therefrom as a plurality of angularly separated beams of different respective wavelengths which are received at respective spatially separated detector positions. In multiplexing mode a plurality of beams of different respective wavelengths are incident on the grating from spatially separated positions and are reflected from the grating as a combined beam to a single output position. Such systems, examples of which are disclosed in U.S. Pat. Nos. 4,111,524, 4,274,706, 4,387,955, European Patent Applications Publication Nos, 0,037,787, 0,067,972, and 0,074,143 and U.K. Patent Application G.B. No. 2,105,489A, can tend to lack versatility and be rather inefficient.
Optical gratings have also been proposed for wavelength selective uses in other devices such as integrated optical circuit devices, see for example U.S. Pat. No. 3,814,498, and other forms of colour selective devices, including holographic devices, have been suggested for various purposes--see for example U.S. Pat. Nos. 3,675,990 and 4,208,637 and U.K. Patent Application G.B. Nos. 2,054,995A and 2,071,866A.